Column by West Dorset MP Oliver Letwin

The prorogation of Parliament did not, as it turned out, have very much effect on Brexit.

But the prorogation did give rise to a Supreme Court judgment, which will have profound implications for the British constitution.

The central question addressed by the courts was whether the use by government of the ancient “royal prerogative” to suspend the operation of parliament was or was not a power that the government had to use reasonably.

The lower courts in Edinburgh and London supported the government’s own view that the courts had no business adjudicating about the reasonableness of the government’s actions in proroguing Parliament, whereas both the higher court in Edinburgh and the Supreme Court in London took the view that prerogative powers of government have to be exercised reasonably.

Once the higher courts had decided that the prerogative power had to be used reasonably, the game was up — because, as the Supreme Court rather tartly pointed out, the government hadn’t given any reasons, much less any good reasons why parliament should be prorogued. But the real importance of the judgment of the Supreme Court lies in the fact that, from now on, the law is that the government has to act reasonably even when it is acting on the basis of its prerogative powers.

When governments are doing things like making treaties or sending British troops into battle (both of which are done under the prerogative power), they will now have to be prepared to defend the reasonableness of their actions in a court of law. So, for example, the reasonableness of the decision to invade Iraq, if it had happened yesterday rather than a few years back, would have been open to challenge in court on the grounds of unreasonableness.

Whatever else Mr Johnson does or doesn’t do, he will certainly have been responsible for a significant adjustment in the relationship between the government, parliament and the courts.